1. Field of the Invention
The present invention relates to a manufacturing method/manufacturing apparatus for manufacturing a seamless pipe while preventing occurrence of thickness deviations, a thickness deviation information derivation apparatus for deriving information about a thickness deviation, and a computer program for realizing a computer as the thickness deviation information derivation apparatus.
2. Description of Related Art
A typical example of a manufacturing method of seamless pipes is a method using a mandrel mill. In this method, first, a billet heated to a required temperature in a heating furnace is pierced and rolled by a piercing mill to obtain a hollow shell, next the hollow shell is elongated and rolled using the mandrel mill, and then sizing rolling to adjust the external diameter and wall thickness is performed using a reducing mill or a sizing mill.
In a seamless pipe manufactured by the above-mentioned process, various types of thickness deviations as shown in FIG. 1A and FIG. 1B may occur. FIG. 1A and FIG. 1B are cross sectional views of a seamless pipe with various types of thickness deviations. FIG. 1A shows cross sections in the direction of the axis of the seamless pipe, and FIG. 1B shows a cross section including the central axis of the seamless pipe. Various factors are listed as the causes of the thickness deviations. Among them, as a factor related to manufacturing equipment, for example, misalignment between the central axis of a plug for use in piercing and the central axis of the billet, or uneven heating temperature of the billet due to insufficient heating in the heating furnace, is listed. A first-order thickness deviation is caused by this factor. On the other hand, a second-order thickness deviation and a fourth-order thickness deviation are caused by the misalignment of the roll position from a set target position in the mandrel mill and the deviation of the radius of a mandrel bar used in the mandrel mill from an appropriate value. Moreover, a third-order thickness deviation and a sixth-order thickness deviation are caused by tool defects in the reducing mill or the sizing mill. Further, as shown in FIG. 1B, there is a deviation of wall thickness changing in the longitudinal direction. A possible cause of this thickness deviation is a deviation due to inclined rolling, such as, for example, the above-mentioned first-order thickness deviation caused by the piercing mill.
In order to prevent occurrence of such thickness deviations, it is necessary to measure the wall thickness of a manufactured seamless pipe, know the state of a thickness deviation, when it is detected, specify the cause of the thickness deviation, and adjust the manufacturing equipment or manufacturing conditions to eliminate the specified cause.
Japanese Patent Application Laid-Open No. 59-7407 (1984) discloses a technique of dividing the cause of a first-order thickness deviation that occurs in an inclined rolling mill, such as a piercing mill, by noticing the degree of twist of a thin portion in a cross section of a seamless pipe in the longitudinal direction. On the other hand, Japanese Patent Application Laid-Open No. 61-135409 (1986) discloses a technique of determining the cause of thickness deviation by measuring the wall thickness of a seamless pipe spirally and analyzing a first-order thickness deviation, a third-order thickness deviation of a 120° cycle, and a second-order thickness deviation of a 180° cycle by Fourier analysis of the measurement results. Further, Japanese Patent Application Laid-Open No. 8-271241 (1996) discloses a technique in which two gamma rays are passed through a seamless pipe, and a first-order thickness deviation is specified based on the difference between the attenuation amounts of the two gamma rays.
In the prior arts mentioned above, the technique disclosed in Japanese Patent Application Laid-Open No. 59-7407 (1984) has the problem that, when there are various types of thickness deviations, it is difficult to determine the form of twist of the first-order thickness deviation. On the other hand, since the technique disclosed in Japanese Patent Application Laid-Open No. 61-135409 (1986) does not disclose a method of analyzing the position of a thickness deviation in the circumferential direction, this technique has the problem that it is impossible to determine an adjustment position where manufacturing conditions are to be adjusted to prevent the thickness deviation, and an adjustment amount. Further, it does not disclose specific measures to prevent a fourth-order thickness deviation of a 90° cycle, a sixth-order thickness deviation of a 60° cycle, etc. Additionally, the technique disclosed in Japanese Patent Application Laid-Open No. 8-271241 (1996) has the problem that it is impossible to divide an even-number-order thickness deviation and an odd-number order thickness deviation.